A Study Of Electron Transport In Graphene Nanostructure

Graphene, a two dimensional material which is formed by arranging carbon atoms in a hexagonal lattice, is attractive to researchers. It has been proved to have many unique properties, such as linear dispersion relation at the Dirac point, integer and half-integer quantum Hall effect, high electron mobility and so on. All of these properties make graphene become the ideal substitute for silicon in semi-conductor industry. In order to open a gap in graphene, we can confine the graphene along a special direction to form graphene ribbon. The confined direction and the width of the ribbon affect the value of the gap. The gap is opened at the Dirac points. As we already known that the Dirac points are degeneracy in graphene. Some researchers have focused on the transport property by differing the two degenerated Dirac points which is useful to make valleytronic device. On the other hand quantum pumping is a powerful method to research the transport in mesoscopic system. This thesis studied the transport property of the bilayer graphene ribbon and generated a pure valley current in graphene by using quantum pumping method. It contains the following chapters.Chapter one gives an introduction of the research background and it included four parts. Firstly we present the widely used methods in laboratory to product graphene and point out the advantage part and disadvantage part of each method. Secondly some graphene properties are discussed in detail. Next the mesoscopic transport theory is explained by comparing with the transpirt theory in macroscopic and microscopic system. At last it is necessary to specify the potential applications of graphene in many aspects of real life.Chapter two is the theory part where the formulas used in the calculation process are derived step by step. Tight-binding model, Green’s function method, scattering matrix are all the favorite theories to the researchers when they solve the transport problem. Quantum pumping theory is different from previous ones where a current flow through the device without a bias. In chapter three we investigate the oscillation property of the transmission probability of the bilayer graphene ribbon with two single layer leads. The DOS of each atom in a unit cell are different. Thus in the bilayer region when the top layer moves relative to the bottom layer, the interlayer interaction strength changes which results in the change of electron transmission probability through the bilayer region. This property can be used to detect the relative movement of the two layers in the order of A.In chapter four we try to generate pure valley current in strain engineered graphene by quantum pumping. The K and K’ valley are degenerate in clear graphene and are separated by the strain induced the vector potential. By adding two anti-parallel magnetic fields a pure valley current is pumped out in the structure. The proposed structure is easily realized in experiment and can be used to make valleytronics.Following the work done in chapter4, we try to separate the two valleys in another method. In chapter five a pure valley current is obtained by using local gapped graphene which is achieved by using BN substrate in specific region. The FM stripes are necessary for pure valley current generation, but they should be in anti-parallel configuration.